Time Assignment Speech interpolation (TASI) communication systems have been known for approximately twenty years. Their function, stated in general terms, is to exploit the fact that during a normal telephone conversation, information is being transmitted only about 35% of the time. TASI communication systems serve to connect a speaker to a transmission line only during those portions of a conversation when speech is actually present. During the other portions of the conversation, the transmission line is connected to another speaker, currently speaking in another conversation. By using time assignment speech interpolation, a given number of transmission lines, such as 24, can usually carry about 48 simultaneous conversations.
A particularly effective TASI system which has met with commercial success is currently manufactured by Electronics Corporation of Israel Ltd. and is described in Published European Patent Application No. 0025465 of Mar. 25, 1981 corresponding to application No. 79301917.5.
All known TASI systems, including that manufactured by Electronics Corporation of Israel Ltd. cannot presently be used in telephone networks having fully compelled interregister multifrequency signalling.
Interregister multi frequency signalling is known as System R-2 line signalling in the CCITT standards and Socotel in the French system. The R 2 system operation is described in detail in Volume VI.3 of the CCITT standards, Orange Book, Section 2, Recommendations Q 411-Q 416, the contents of which are incorporated herein by reference as background source material. Briefly described, the R 2 system involves an in-band multifrequency code with forward and backward fully compelled signalling, for call set up control signals, i.e. interregister signalling. Line signalling is provided by out of band signalling such as E and M wire signalling and only indicates the change of states from one signalling state to another. For example, the transition from the "idle" state to the "seized" state is indicated by an out of band transition from TONE-OFF on the outgoing line and an unchanged TONE-ON status on the incoming line.
A number of functions of time assignment speech interpolation systems, such as the ECI system, are incompatible with interregister multifrequency signalling. The use of an Echo Suppressor during signalling has been found by applicants to at least partially block the interregister signalling due to the simultaneous presence of signals in both directions. Applicants have also been found that the provision of a 6 db attenuation as part of the echo suppression in the receiver interferes with signalling receipt in the R-2 signalling protocol.
Applicants have also found that the delay employed in TASI apparatus acts to extend the time-out period assigned to fully compelled interregister multifrequency signalling in the R-2 protocol.